The present invention relates to an improved optically read recording medium, and to methods for making such a medium. The recording medium of this invention is well suited for use as a video or audio disc.
In the past, a variety of techniques have been used to manufacture optically read recording media, including both mechanical and photolithographic techniques. Conventional video and audio discs (such as compact discs) are typically made by a mechanical pressing operation in which 1/4 wavelength pits are physically stamped into the recording medium. Commonly assigned U.S. Pat. No. 4,423,137 discloses a method for fabricating recording media photolithographically. This photolithographic approach eliminates the significant drawbacks encountered with conventional stamping techniques.
Although the photolithographic approach described in U.S. Pat. No. 4,423,137 is well suited for a wide variety of applications, it produces a recording medium which does not rely on destructive interference during the reading operation. This is because the recording medium described in this patent includes transmissive holes which pass the reading beam rather than reflecting it with a suitable phase offset. There are reading devices, such as certain compact disc players, which utilize phase differences between the reading beam reflected from various portions of the recording medium for control purposes.
The present invention is directed to an improved recording medium which retains the important advantages of photolithographic processing, while providing a recording medium that relies on phase cancellation techniques for the reading process.